1. Field of the Invention
This invention relates in general to multitrack recording apparatus and in particular to apparatus for preventing flutter and skew from occurring during the playback of recorded information signals.
2. Description Relative to the Prior Art
Flutter: In the recording of information signals on, say, a magnetic medium such as magnetic tape, it is usual to move the magnetic tape at a given speed during such recording, thereby to lay the information signals along the length of such tape. Then, during playback of the information signals recorded on the tape, the tape is moved relative to a playback head at the same given speed, thereby to provide faithful reproduction of the recorded information signals. Any cyclic change in the playback tape-to-head speed vis-a-vis the tape-to-head speed employed during recording manifests itself as flutter affecting the playback of the recorded signals.
Skew: When time-coherent information signals are recorded on a magnetic medium such as magnetic tape in several adjacent tracks thereof, it is necessary that the time coherence be preserved during signal playback, for otherwise relative distortion will occur among the various information signals. Typically, time-coherent information signals are recorded in adjacent tracks of a magnetic tape by means of a conventional multitrack magnetic head having a succession of aligned transducer gaps. The alignment of gaps constitutes a "gap line" and, during recording, the gap line has an azimuthal angle of, say, zero degrees with respect to the tape width. During playback, a multitrack head, with transducer gaps also arrayed along a gap line, coacts with the tape and, if the tape playback head gap line has the same (zero degrees) azimuthal angle as was employed during recording, distortion-less playback of the respective information signals results. As it turns out, however, two things can go awry and cause distorted playback: first, the azimuthal angle of the playback head gap line can be different from the record head gap line azimuthal angle, thereby giving rise to static skew; secondly, the tape during playback can cyclically yaw about an axis perpendicular to the plane of its recording surface, thereby giving rise to dynamic skew.
Admittedly, various signal processing techniques (generally involving delay lines and the like) have been disclosed which are directed to the elimination of flutter and static and dynamic skew from signals. . . after such conditions actually occur. In contrast with such techniques, there are also a number of known techniques which are directed to procedures for precluding flutter, and static and dynamic skew, from ever occurring in the first place. It is with respect to these latter techniques that the present invention is directed.
Falling into the category of those techniques for precluding the very existence of flutter and/or skew are the techniques disclosed in the following references:
U.S. Pat. No. 2,656,419, issued Oct. 20, 1953, teaches the recording of a signal of a frequency f along the length of a magnetic tape and, during playback, phase-comparing the recorded signal with a stable reference of frequency f, thereby to discern any doppler error in the recorded signal. A signal corresponding to the doppler error is then employed, in a dynamic mode to position the playback head to cancel, continuously, the error, and thereby preclude the existence of flutter.
U.S. Pat. No. 2,937,239, issued May 17, 1960, discloses a skew prevention technique in which reference signals are recorded in outbound tracks of a magnetic tape. The reference signals are played back, and phase-compared, to produce an error signal which is used to servo rotationally the playback head about an axis through the face of the head. Thus, any skew which starts to occur is immediately cancelled. (U.S. Pat. No. 3,204,228, issued Aug. 31, 1965, incidentally, employs reference signals recorded in outbound tracks much in the way of U.S. Pat. No. 2,937,239, but for purposes of producing respective signal processing delays for cooperation with skewed tracks between the outbound tracks.)
U.S. Pat. No. 3,526,726, issued Sept. 1, 1970, combines the goals of U.S. Pat. Nos. '419 and '239 in a magnetic head support employing four piezoelectric benders (also known in the art as "bimorphs"). Outbound tracks of a magnetic tape have reference signals recorded therein. The recorded reference signals during playback thereof are not compared with each other, but rather are compared with signals from a reference signal source. Such comparisons result, in the case of skew, in the production of oppositely-polarized signals which are applied to respective pairs of benders. One signal causes a pair of benders to skew the playback head one way; the other signal causes the other pair of benders to skew the head the other way. Absent the reference signal source, the apparatus of U.S. Pat. '726 will not, and indeed cannot, work to preclude skew. In the case of flutter, the signal comparisons which were indicated above produce similarly polarized signals which are applied to the respective pairs of benders. Since the signals are similarly polarized, the benders similarly translate the playback head to preclude flutter.
U.S. Pat. No. 3,787,616, issued Jan. 22, 1974, discloses the use of a single piezoelectric bender as a cantilever to which a magnetic playback head is attached. Sync signals derived from magnetic tape moving relative to the head are phase-compared with locally generated signals to produce error signals for flexing the bender to correct any time-base error caused by variation in the playback speed of the tape vis-a-vis the tape record speed.
Other art which has been examined and which serves as a backdrop for the present invention is as follows:
U.S. Pat. No. Re. 20,213, reissued Dec. 22, 1936 PA1 U.S. Pat. No. 2,858,373, issued Oct. 28, 1958 PA1 U.S. Pat. No. 3,183,516, issued May 11, 1965 PA1 U.S. Pat. No. 3,706,861, issued Dec. 19, 1972